The present invention relates to an improved process of extraction of uranium from phosphoric acid and in particular uranium VI from phosphoric acid especially strong phosphoric acid using a selective synergistic extractant mix of an organo-phosphorous acid and a neutral extraction agent.
Phosphates are well known to be an important sustainable secondary source of uranium. While methods of uranium extraction such as ion-exchange, membrane separation and precipitation are well known, the method of solvent extraction of uranium has been found to be a successful process for industrial recovery of uranium from phosphates.
It is well known to carry out uranium recovery from wet process phosphoric acid obtained via sulfuric acid route. There are various known solvent extraction processes, which include the OPPA process, which utilizes octyl pyrophosphoric acid as the extractant. This solvent is inexpensive but is found to be highly unstable especially during the exposure to HF, which is needed for back extraction.
It is also known to carry out solvent extraction of uranium using the OPAP process which involves the use of octyl phenyl acid phosphate consisting of mixture of mono octyl phenyl phosphoric acid (MOPPA) and di-octyl phenyl phosphoric acid (DOPPA). It is characterized with high extraction of uranium. However, it has limited solubility in diluents like kerosene and stability although comparatively better than OPPA is not very high. Moreover, the selectivity is also not very high.
U.S. Pat. No. 4,302,427 discloses the uranium extraction using the solvent mixture of di-(2-ethyl hexyl) phosphoric acid (D2EHPA) and tri-n-octyl phosphine oxide (TOPO) in a phosphoric acid immiscible organic solvent. U.S. Pat. No. 4,778,663 discloses a further process of recovery of uranium from wet process phosphoric acid involving the use of a combination of dialkyl phosphoric acid and mixture of trialkyl phosphine oxides. Such processes are characterized with low distribution ratio of uranium and essentially requires the use of relatively expensive extractant component, which is TOPO.
U.S. Pat. No. 4,238,457 discloses the use of a first stage extraction using organic solvent consisting essentially of dialkylphosphoric acid and trialkylphosphine oxide dissolved in an inert and unreactive organic solvent followed by a second stage re-extraction using ammonium hydroxide and ammonium carbonate. Such a process is cost-extensive and yields a product of low purity.
Moreover, solvent extraction processes that have been reported as successful for extraction of hexavalent uraniumxe2x80x94the form in which uranium is generally present in phosphoric acidxe2x80x94involve extraction by a mixture of an organo-phosphorous acid with a neutral synergist. Thus, di-(2-ethyl hexyl) phosphoric acid (D2EHPA)-tri-n-butyl phosphate (TBP), D2EHPA-di-butyl butyl phosphonate (DBBP) and D2EHPA-tri-n-octyl phosphine oxide (TOPO) combinations have been reported. Results on DOPPA and di-nonyl phenyl phosphoric acid (DNPPA) have also been reported with their mono-ester components for extraction of tetravalent form of uranium.
It would be evident from the above that for extraction of uranium from phosphoric acids whether U-IV or U-VI is known to involve use of the combination of a organo-phosphorous acid as the basic component and a synergistic agent. Although this is generally known, it is extremely difficult to ascertain the specific combinations of phosphoric acid and the synergistic extractant agent and conditions for extraction which could achieve the desired extraction and which would be industrially applicable and be cost effective.
Importantly, lack of clear knowledge on the mechanism of extraction even for known extraction systems makes it extremely difficult to reach to a selective combination which would provide for better extraction and would also be cost effective. There are instances where apart from selection of the acid/agent for extraction, the proportion in which the two compounds acid and the agent are used determine whether it would have a synergistic effect or an antagonistic effect. This made it all the more difficult to identify the selective combination of extractant, which would provide the improved extraction by using a cost effective process. Added to the above, the knowledge of extractant for extraction from weak phosphoric acid does not have direct relevance on the selection of the extractant for extraction from strong acids. This made it difficult to provide extraction systems for strong phosphoric acids.
It is thus the basic object of the present invention to provide for an improved process for recovery of uranium both from weak and strong phosphoric acids using a stable and relatively cheap extractant system.
Another object of the present invention is directed to provide for an improved process of recovery of U-VI from strong phosphoric acid using a novel selective combination of an organo-phosphorous acid and a neutral synergistic agent.
Yet further object of the present invention is to provide for a synergistic combination of a organo-phosphorous acid and a neutral synergistic agent which would provide for improved recovery of U-VI from phosphoric acid by way of a simple, industrially applicable and cost-effective process.
Yet further object of the present invention is directed to provide for a synergistic combination of the organo-phosphorous acid and TOPO/DBBP which would provide for selective and high purity yield/extraction of U-VI both from weak as well as strong phosphoric acids.
Thus according to the present invention there is provided an improved process for extraction of uranium from wet process phosphoric acid comprising:
a. steps of extraction comprising contacting said acid with a selective synergistic extractant system of di-nonyl phenyl phosphoric acid (DNPPA) and a neutral agent selected from di-butyl butyl phosphonate (DBBP) and tri-n-octyl phosphine oxide (TOPO); and
b. recovering the uranium values from the loaded organic phase.
In accordance with one preferred aspect of the present invention the improved process for extraction of uranium from weak phosphoric acid (containing 26-30% P2O5) comprises:
a. steps of extraction comprising contacting said acid with said selective synergistic extractant mix of di-nonyl phenyl phosphorous acid (DNPPA) and di-butyl butyl phosphonate (DBBP) after appropriate dilution; and
b. recovering the uranium values from the loaded organic phase.
In accordance with another preferred aspect of the present invention the improved process for extraction of uranium from strong phosphoric acid (containing 30-55% P2O5) comprises:
a. steps of extraction comprising contacting said acid after appropriate dilution with a selective synergistic extractant mix of di-nonyl phenyl phosphoric acid (DNPPA) and tri-n-octyl phosphine oxide (TOPO); and
b. recovering the uranium values from the loaded organic phase.
Preferably, in the above discussed processes of the present invention the selective mole ratio of the extractant mix comprising of said acid DNPPA and the agent DBBP/TOPO comprise mole ratio of 0.05 to 0.20 M DBBP : 0.2 M DNPPA and 0.05 to 0.155 M of TOPO:0.2 M DNPPA. It was found that the extraction increased with the increase in the level of the synergistic reagent used. However, it was identified that the optimum mole ratio of synergistic reagent (DBBP/TOPO) with extractant is 1:2.
Moreover, in the above disclosed processes of the invention, both for the weak phosphoric acid as well as the strong phosphoric acid, prior to said step of extraction the acid is subjected to pre-treatment comprising steps of separation of suspended solids, separation of humic matter and oxidation. For the purpose, the humic matter was characterized by infrared (IR) spectra. It was found that long chain polymeric flocculent used for separation of suspended solids from freshly produced acid was also effective in initial removal of organic matter. Residual humic matter, not removed by clari-flocculation, was removed by carbon adsorption. As an alternative xe2x80x9csolvent scrubbingxe2x80x9d method can also be used. After humic matter separation, oxidation with air sparging was followed by polishing oxidation with hydrogen peroxide carried out in stages. Clear oxidized acid was subjected to solvent extraction.
Preferably, in the above process of the invention, after the extraction is effected, the barren acid is subjected to post-treatment for entrained solvent recovery. Different methods of stripping such as ammonium carbonate, HF or strong H3PO4 under reducing conditions can be followed.
In accordance with another aspect of the invention it was found that the phase continuity in the mixer played an important role, especially for MGA. With aqueous continuous operation, entrainment exceeded 300 mL/m3, whereas with organic continuous, it was 130-260 mL/m3. Use of a coalescer with a residence time of 13 min reduced entrainment to 18-26 mL/m3.